Driver output stage circuits usually consist of a primary side circuit and a secondary side circuit, which are isolated from each other. The primary side is usually grounded. The isolated secondary side requires an isolated supply. Within the secondary side, commands from the primary side are usually treated to have the right shape—at least to have the right voltage levels—to control the power semiconductor. For driving the gate of a power transistor such as a MOSFET (metal-oxide-semiconductor field-effect transistor), IGBT (insulated gate bipolar transistor), JFET (junction field-effect transistor), or HEMT (High Electron Mobility Transistor), driver output stages conventionally include an output stage for amplification of input signals entering the driver output stage circuit or signals generated within the driver output stage circuit. The driver output stage of the driver output stage circuit typically includes an emitter follower stage in a push-pull configuration. The input signal or the signal generated within the driver output stage circuit applied to the driver output stage has the desired voltage magnitude and shape over time, which is intended to be applied to the gate of the power semiconductor. The driver output stage should amplify in current only, and should maintain the voltage shape or level. The driver output stage is typically supplied by one (e.g. 15V) or two supply voltages (e.g. +15V, −5V). The high and low levels of the input signal are usually equal to the positive and negative supply voltages, respectively. In the case of n-channel MOSFETs or n-channel IGBTs, in the on-state, the positive supply voltage of the driver output stage ideally is switched to the gate of the power transistor without voltage drop. In the off-state, the negative supply voltage of the driver output stage ideally is likewise switched to the power transistor gate without voltage drop. In case of p-channel MOSFETs, the polarities are inverted according to the control characteristics of those devices. For normally-on power transistors, the voltage levels for the ‘on’ and ‘off’ states may be 0V and −15V, or whatever negative voltage (e.g. −5V, −20V) is required for the device, respectively.
If the driver output stage includes an emitter follower circuit, there will be a voltage drop corresponding to the forward voltage of a base-emitter diode of the emitter follower. For Darlington configurations, the voltage drop can be two or three times larger depending on the number of Darlington stages. Such voltage drops are especially active when peak currents flow into or out of the gate of the power semiconductor and through the driver output stage. Such currents appear during transitions e.g. turn-on and turn-off and during voltage changes at the load terminals of the power semiconductor or in the case of voltage changes at the load terminals of the power semiconductor, i.e. drain or collector voltage transitions at the power semiconductor. Such drain or collector voltage transitions can be caused by reasons external to the power device such as short circuit of the load occurring in conduction mode of the power semiconductor. Another example occurs by diode current commutation because of reversing the load current at the phase output of a half-bridge circuit within a power inverter, etc. Unless mitigated, these voltage drops in the output stage of the driver circuit result in higher switching losses or unreliable operation of the power semiconductor under certain short circuit conditions or under current commutation from the freewheeling diode. The advantage of the emitter follower circuit despite the inherent voltage drop concerns is excellent response time on the input signal.
A driver output stage which maintains excellent response time but prevents voltage drop between the input and output of the driver output stage is therefore desired.